Recently, direct current (DC) signal, such as DC voltage or DC current is widely used as power supply for many electronic products. Switch mode power supply may be employed to provide DC voltage or DC current. Switch mode power supply may comprise energy storage element such as inductor or transformer. Taking flyback converter as one example, a switch is coupled to a primary winding of a transformer, and the switch is turned ON/OFF to regulate an output voltage or an output current via storing and transferring energy to a secondary winding of the transformer alternately.
With development of electronic technology, it is believed that the development trends of switch mode power supply would be high integration and high reliability. Generally, control circuit for controlling switch mode power supply may be integrated on an Integrated Circuit (IC), and a control IC is configured to provide general control function to turn ON and/or turn OFF a switch and is configured to provide protection such as over current protection, over voltage protection or over temperature protection.
The control IC may have a current sense pin and a voltage sense pin, the current sense pin is configured to receive a current sense signal for over current protection and the voltage sense pin is configured to receive a voltage sense signal for over voltage protection. The control IC also needs over temperature protection. Traditionally, there are two major methods providing over temperature protection. One method is to integrate temperature sense circuit and protection circuit on the control IC. But there are many disadvantages such as larger chip area, lower sensing accuracy, lower reliability and lower flexibility and having a fixed over temperature protection point which can not vary for different application. The other method is to add a temperature sense pin and detect temperature through external circuit, but the disadvantage is that larger package size is needed.